Aquatic Insects of Michigan

by Ethan Bright, Museum of Zoology Insect Division and School of Natural Resources and Environment
University of Michigan

Boyeria McLachlan - Fawn and Slate Darners

Both North American species - B. grafiana (Fawn darner) and B. vinosa (Slate darner) - are found in Michigan. A late-emerging genus in our area, nymphs emerge usually in late June through August. Adults are large, brownish, and agile dragonflies that fly close to the surface of woodland streams and rivers (Needham et al. 2000). Adults are distinctive among other aeshnids by wing venation characters (as described in the generic-level key for Aeshnidae) and two rounded light spots on the pterothorax. Nymphs are distinctive among our other aeshnids by the distinctly angulate posterolateral head margins, and a large, conspicuous, light blotch on the middorsum of Ab8.

B. vinosa is commonly encountered in quieter sections of streams, rivers, and rarely in wave-swept portions of lakes (EB, pers. obs.) throughout Michigan. Nymphs are collected amongst the accumulations of woody debris or organic leafy matter as well as under stones and submerged objects (Walker 1958). Williamson (1932) stated that B. vinosa "is by all odds the most abundant and most widely distributed lotic Anisoptera east of the Great Plains." Sampling from numerous localities in Michigan commonly yield two different size classes, indicating a two-year life cycle for nymphs (EB pers.obs.). Exuviae usually are found on exposed logs and rocks as well as vegetation in and nearside the stream. B. grafiana only rarely has been collected in Michigan (Bright 1997).. Along the shore of Georgian Bay, Walker (1958) found nymphs along the rocky shores of inland lakes or stream rapids, and exuviae were collected "on rocks, tree trunks or boathouses near the water and the tenerals fly first to a sheltered shady spot, such as a tree trunk, branch, or a cottage verandah." Although Walker stated that B. grafiana is principally a lake species in Ontario, and usually does not occupy the same sites as B. vinosa, Perry (1977) found both species in tributaries of the Grand River in Ohio. Nymphs of B. grafiana "were collected from Pierson Creek on 2 August 1974...[from] beneath small stones in the more rapidly flowing portions of the main stream. They were placed in a simulated creek-bed set up in a laboratory aquarium and supplied with living mayfly and stonefly nymphs and crane fly larvae for food. A female grafiana emerged on 13 August, 1974, followed by a male on 22 August. Another male grafiana was taken in flight above Pierson Creek on 16 August, 1974."

Taxonomic references (Wright 1949, Walker 1958, Louton 1982, Needham et al. 2000, Tennessen 2019)

Adults

1a	a. Wings each completely hyaline at the base, or with a minute trace not nearly reaching the basal antenodal cross vein	Boyeria grafiana Williamson
	b. Pale middorsal spots on Ab4-7 conspicuous
1b	a. Wings each with a brown spot reaching, or nearly reaching, the antenodal crossvein	Boyeria vinosa (Say)
	b. Pale middorsal spots on Ab4-7 absent or very obscure

Mature Nymphs

1a	a. Epiproct usually emarginate, may be shorter than paraprocts	Boyeria vinosa (Say)
	b. Ratio of prementum length : basal width < 3.0
	c. Movable hook length usually < 1.5 mm
1b	a. Tip of epiproct pointed	Boyeria grafiana Williamson
	b. Prementum longer, ratio of prementum length : basal width > 3.0
	c. Movable hook length > 1.5 mm

References

Bright E. 1997. New larval Odonata records for Michigan. Williamsonia 1(2):10.
Louton JA. 1982. Lotic dragonfly (Anisoptera: Odonata) nymphs of the southeastern United States: identification, distribution and historical biogeography. Ph.D. thesis, University of Tennessee, Knoxville. 357 + xvii.
Needham JG, Westfall MJ, May ML. 2000. Dragonflies of North America, Revised Edition. Scientific Publishers, Gainesville, Florida, USA. xv + 939 p.
Perry TE. 1977. Boyeria grafiana, a rare Ohio dragonfly (Odonata: Aeshnidae). Great Lakes Entomologist 10:159-161.
Tennessen KJ. 2019. Dragonfly nymphs of North America: An Identification Guide. Springer International Publishing, Cham, Switzerland: xiv + 620 p.
Walker EM. 1958. The Odonata of Canada and Alaska, Vol. 2. University of Toronto Press: Toronto. xii + 318.
Williamson EB. 1932. Dragonflies collected in Missouri. University of Michigan Museum of Zoology, Occasional Paper 240. 40 pp.
Wright M. 1949. Notes on nymphs of the dragonfly genus Boyeria. Journal of the Tennessee Academy of Science 24(3):213-215.

Page last edited: April 19, 2020 (EB)